In a wavelength multiplexing optical system where several signal lights with different wavelengths are transmitted in an optical fiber, a band pass filter in needed to extract signal light with a desired wavelength from the transmitted signal light in a receiving unit for signal light. However, when a transmitted center wavelength of the band pass filter is fixed, the transmission loss of the band pass filter may be highly increased due to a variation of wavelength in the signal light caused by a time dependent variation or temperature variation of a light source for transmission. Therefore, it is desirable that a band pass filter used in the receiving unit is a wavelength-selective optical device whose transmitted center wavelength can be changed.
Japanese patent application laid-open No. 7-198936 (1995) discloses a wavelength-selective optical device which is composed of optical filter that dielectric multilayer film is coated on convex glass, the optical filter being inserted between two opposing optical fibers, and a piezoelectric actuator which changes the position of the convex glass to change the incidence angle of light to the filter film so as to change the transmitted center wavelength of the filter film.
Japanese patent application laid-open No. 5-005805 (1993) discloses a wavelength-selective optical device which is composed of an optical filter plate and an optical path correcting plate which are inserted between two opposing optical fibers, wherein the incidence angle of light is changed by the rotation of the optical filter plate to change the transmitted center wavelength and the position change of lightbeam due to the rotation of the optical filter plate is corrected by the rotation of the optical path correcting plate.
Further, Japanese patent application laid-open No. 56-162701 (1981) discloses a wavelength-selective optical device which uses a property that the rotation angle of a rotatory material is varied depending upon a wavelength of light. Also, Japanese patent application laid-open No. 8-110487 (1996) discloses a wavelength-selective optical device where input light is supplied to a polarization separating prism to transmit only specific linearly polarized light, and an optical filter plate, a quarter-wave plate and a reflection plate are inserted on the optical path of the transmitted light, wherein the optical filter plate is rotated to change the transmitted center wavelength. In this device, light transmitted through the polarization separating prism is transmitted by two times, each time in the different direction, through the optical filter plate. Thereby, a position change of optical pass is corrected. Therefore, the device has a merit that the coupling loss between input and output fibers by the rotation of the optical filter plate is not changed. Also, it has a further merit that a steep transmission loss spectrum with a narrow half width can be obtained since light is transmitted by two times through the optical filter plate.
However, the conventional wavelength-selective optical devices have several problems described below. In the device disclosed in Japanese patent application laid-open No. 7-198936, there is a problem that the movement of the optical filter causes a change in the optical path of light beam and the coupling loss between input and output fibers is therefore increased. Further, the transmittance of the optical filter of dielectric multilayer film is generally varied depending upon a change in the polarization state of incident light with an increase in incidence angle of light. Namely, with an increase in incidence angle of light, the polarization-dependent loss (PDL) of the optical filter due to the change in the polarization state of incident light is increased.
In the device disclosed in Japanese patent application laid-open No. 5-005805, there is a problem that the composition is complicated since the optical path correcting plate is needed other than the optical filter plate. Also, there is a further problem that the adjustment control is complicated since the rotation angle of the optical path correcting plate needs to be controlled corresponding to the rotation quantity of the optical filter plate. Further, like the above device, there is a problem that PDL is increased with an increase in incidence angle of light to the optical filter and optical path correcting plate.
In the device disclosed in Japanese patent application laid-open No. 56-162701, the resolution capability for wavelength separation is no more than a few nano-meters due to the wavelength dependency of rotation angle in the rotatory material. Therefore, it is difficult for such a wavelength-selective optical device to extract only signal light with a specific wavelength from signal lights which are arranged with an interval of about 0.1 to 1 nm in a wavelength multiplexing optical communication system.
In the device disclosed in Japanese patent application laid-open No. 8-110487, when input light is circularly polarized, elliptically polarized or non-polarized, there is a problem that loss at the polarization separating prism is increased. Also, there is a further problem that loss at the polarization separating prism is varied with time since the polarization state of signal light to be transmitted through the optical fiber is varied with time. Further, the composition is complicated since the number of components is so large.
Also, in all the conventional wavelength-selective optical devices, optical elements, such as an optical filter, need to be mechanically operated. Therefore, it is difficult to obtain a high reliability.